Powertrain, generic · Transmission

P0802 Transmission Control System MIL Request Circuit/Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

On older vehicles with a separate transmission module and no MIL message on the CAN bus, the transmission module signals 'MIL on' over its own wire to the engine module. P0802 checks this wire for an open circuit. GM's Duramax tables of 2004 and 2005 (Allison 1000) describe it: at key-on the transmission module briefly pulls the line active like a bulb check; if the engine module sees no off-to-on transition between 0.125 and 7 s after ignition, the line is treated as open – type B, MIL after two trips. Only then is P0700 evaluated. The wire itself must be monitored because an open line would stop transmission emission faults from lighting the MIL. Newer vehicles send the request over CAN, where faults appear as U codes.

Likely causes

MIL request wire or connector between the modules open or corroded
Transmission module without power or ground (fuse, relay)
Module output or input faulty
Wrong or retrofitted transmission module without this line

Symptoms

  • Check engine lamp on
  • Transmission faults no longer light the MIL
  • Usually no change in driving

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–400
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transmission control moduleengine control modulemil request wiring

Related codes

P0700P0650U0101P0613

References: gsi.ext.gm.com, gsi.ext.gm.com.